I got the same issue with my iPhone 5s 16 GB, received as a gift for my birthday last summer (so, still in warranty). I had a previous model, iPhone 5 which worked wonderful, and stayed in stand by almost 4 days! But with this new one, 5s, after update to IOS 8.1.2 it does not stay in stand by mode more than 14 hours. It is awful, since I need the phone to stay in this feature at least as advertised in specifications, respective 250 hours. When someone by something from apple, the product must comply with the characteristics provided by the producer! This is a problem of trust with or within apple from now on. What's wrong with apple?
More than that, I went with the phone to a authorised service by apple in my country, and guess what? They told me the error or defect does not manifest...even they kept the phone just two days an a half in service. They said they have specific test that suggests there is no issue with the battery autonomy. But this is not a battery issue is a software one!!!
And even more, How can you test the product for stand by mode in less than 250 hours as specified in characteristic?
After all, then they told me to call myself apple and discuss with them the issue. Therefore, I tried to do that however the robot directed me to contact my mobile service provider, therefore a dead end. I did not give it up, and I work around this other apple communication gap, and got in contact with someone from laptops department. I explained to him my issue, and the fact I cannot go to my service provider because the iPhone is a gift therefore not bought through a service provider, and in that case his recommendation was to send it back to a friend in USA or somewhere in Europe IOT change it by him through an Apple retailer or original store.
I got back to the authorised service by apple and ask them why they cannot send my phone for test or replacement directly to apple. Their answer was that it has to be an agreement between apple and me personally IOT to allow them to do that!?! That's amassing... considering the fact that I cannot even get in contact with someone from apple responsible directly for iPhones issues!
That's a shame how apple treat in this way its costumers in country that they have not a direct retailer or original store! Even when the product is still in warranty status!!
So, long talk for nothing... Anyway, is there anyone that can help me at least to get back to IOS 7, respective delete IOS 8.1.2 from my phone forever and get back to stand by for more than 24 hours?
I tried all the advices saw here to stop working that or modify the other, however nothing works for my iPhone!...